Referring to FIG. 1, a turbine engine 10 generally includes a compressor section 12, a combustor section 14, a turbine section 16 and an exhaust section 18. In operation, the compressor section 12 can induct ambient air and can compress it. The compressed air from the compressor section 12 can enter one or more combustors 20 in the combustor section 14. The compressed air can be mixed with the fuel, and the air-fuel mixture can be burned in the combustors 20 to form a hot working gas. The hot gas can be routed to the turbine section 16 where it is expanded through alternating rows of stationary airfoils and rotating airfoils and used to generate power that can drive a rotor 26. The expanded gas exiting the turbine section 16 can be exhausted from the engine 10 via the exhaust section 18.
The exhaust section 18 can be configured as a diffuser 28, which can be a divergent duct formed between an outer shell 30 and a center body or hub 32 and a tail cone 34. The exhaust diffuser 28 can serve to reduce the speed of the exhaust flow and thus increase the pressure difference of the exhaust gas expanding across the last stage of the turbine. In some prior turbine exhaust sections, exhaust diffusion has been achieved by progressively increasing the cross-sectional area of the exhaust duct in the fluid flow direction, thereby expanding the fluid flowing therein.
It is preferable to minimize disturbances in the exhaust diffuser fluid flow; otherwise, the performance of the diffuser 28 can be adversely affected. Such disturbances in the fluid flow can arise for various reasons, including, for example, boundary layer separation. If fluid flow proximate a diffuser wall (the boundary layer) separates from the wall, there is a loss in the diffusing area and an increase in total pressure loss resulting in reduced pressure recovery. Generally, the larger the angle of divergence in a diffuser, the greater the likelihood that flow separation will occur.
One approach to minimizing flow separation is to provide a diffuser with a relatively long hub. A long hub can maximize performance by delaying the dump losses—flow losses that occur at the downstream end of the hub/tail cone—to a point when the exhaust gases are traveling at a lower velocity, thereby minimizing the strength of the tail cone's wakes in the flow. However, a long hub presents a disadvantage in that it can make the engine design more complicated and expensive. For instance, a longer hub typically requires two rows of support struts 36—one in an upstream region of the hub 32 and one in a downstream region of the hub 32, as shown in FIG. 1. These support struts 36 can increase cost and the risk of material cracking due to thermal mismatch between inner and outer flowpath parts or vibratory loads. Further, long hubs can pose challenges in instances where available space is limited.
Another approach to minimizing flow separation losses is to provide a diffuser with a relatively short hub length followed by a reduced divergence angle. This approach can minimize cost by, among other things, requiring only a single row of support struts. However, diffuser performance may suffer because this design can often lead to high dump losses from having the hub end (sudden expansion) further upstream in the diffuser where the flow velocities are higher. To avoid a second set of struts, associated tail cones are often steep, causing wakes to form in the flow downstream of the tail cone which can continue to grow downstream.
Thus, there is a need for an exhaust diffuser that can achieve the performance benefits of a long hub design while enjoying the reduced cost and risk of a short hub design.